The role of calcium in insulin action. IV. Mechanism of insulin resistance in adipose tissue of obese (ob/ob) mice and old Wistar rats.

The in-vitro antilipolytic response to insulin and procaine hydrochloride by adipose tissue from young rats (150 - 180 g) and lean mice has been compared with that from aged Wistar rats (600 g) and obese (ob/ob) hyperglycaemic mice. 1. The adipose tissue from the obese mice showed diminished responsiveness to insulin and to procaine hydrochloride. Response to these agents, however, was restored by prewashing the tissue, suggesting that the apparent resistance in this tissue reflected saturation of the insulin receptors to endogenous insulin. 2. In adipose tissue of old Wistar rats the antilipolytic effect of insulin was also impaired, but this was not restored after extensive washing. Unlike adipose tissue ghosts prepared from young rats, insulin did not decrease the binding of calcium to ghost membrane preparations from old rats. Neither did insulin inhibit the adrenaline stimulated 45 calcium efflux from perifused isolated fat cells prepared from old rats. These results suggest that the insulin response of fat cells from old rats is impaired because of a defect either in their insulin receptors or in their post-receptor responses. 3. Procaine-hydrochloride, however, when added to the medium perifusing fat cells of these old rats inhibited the adrenaline stimulated lipolysis, reduced the Ca efflux and decreased the binding of Ca to fat cell ghosts; as it did with similar preparations of young rats. Thus the cells from old rats still show full post-receptor responsiveness to an insulin-like stimulus, provided the stimulus for such a response is given at a point beyond the insulin receptor itself. The results suggest that the insulin resistance observed in old rat fat cells may be related to some by deficiency in the insulin receptors, possibly due to their lower replacement with age.